The present invention relates to an arithmetic operation apparatus which uses solar cells as a power source and which is used with low power consumption for a compact electronic calculator and/or an electronic timepiece.
Along with development of large scale integrated circuits (LSI), many compact electronic calculators, electronic timepieces or the like, have electronic circuits comprised of a single LSI. The current consumption of each such LSI may be made lower than 10 .mu.A at a voltage of 3 V, or 1.5 V when the LSI has a complementary metal oxide semiconductor (CMOS) structure. The development of LSI techniques allows new power source means to be used. Various types of dry cells have been used as the power source means for compact electronic equipment, such as compact electronic calculators and electronic timepieces. More particularly, a compact and lightweight dry cell which has a low current production has been used along with the low current consumption of the LSI. In the alternative, a solar cell has been recently used for the power source for the above-mentioned arithmetic operation apparatuses.
FIG. 1 is a block diagram showing the general arrangement of a compact electronic calculator using solar cells as the power source. In FIG. 1, solar cells 1 are connected in series with each other. The negative terminals of the solar cells 1 are commonly connected to a V.sub.DD terminal 4 of an LSI 3 through a resistor 2. The V.sub.DD terminal 4 supplies a power source voltage V.sub.DD (negative voltage). The positive terminals of the solar cells 1 are commonly connected to a V.sub.SS terminal 5 of the LSI 3. The V.sub.SS terminal 5 supplies a reference voltage V.sub.SS (positive voltage). A bypass capacitor 6 is parallel-connected between the V.sub.DD terminal 4 and the V.sub.SS terminal 5. A series circuit of light-emitting diodes (LEDs) 7 and 8 is also parallel-connected between the V.sub.DD terminal 4 and the V.sub.SS terminal 5. The LEDs 7 and 8 serve to prevent a surge voltage from being applied across the LSI 3 when the intensity of light incident on the solar cells 1 is very high. The resistor 2 and the capacitor 6 are arranged to prevent variations in the voltage applied across the LSI 3 even if the intensity of light incident on the solar cells 1 is abruptly changed.
The LSI 3 includes electronic circuits such as an arithmetic and logic unit, an oscillator and a display driver so as to execute a series of operations (i.e., from the execution of various types of operation to the generation of a display signal for displaying the operation results). A keyboard 9 and a liquid crystal display (LCD) 10 are connected to the LSI 3.
FIG. 2 is a graph showing general voltage-current characteristic curves and the load characteristics of the LSI 3, using the illuminance (Lux) as a parameter. As may be apparent from the graph, the power capacity of the solar cells increases when the illuminance increases. However, the power capacity of the solar cells decreases when the illuminance decreases. Therefore, when a solar cell is used as a power source, a constant voltage cannot be obtained, unlike a case in which any cell other than the solar cell, such as a silver-oxide cell, is used. An output voltage of the solar cell greatly changes in accordance with changes in the ambient illuminance and the orientation of the solar cell with respect to the light source.
A compact electronic calculator of FIG. 1 configuration has two statuses: a control signal reception ready status (i.e., key signal waiting status wherein the LSI 3 awaits a key signal from the keyboard 9); and an operating status wherein any desired operation may be executed after the key signal is received. Although some compact electronic calculators do not have these two statuses, a compact electronic calculator having the two functions is illustrated for the sake of simplicity. In general, the current consumption of the LSI 3 in the operating status is greater than that in the key signal waiting status. This is because the number of active circuits in the operating status is greater than that in the key signal waiting status.
A line L1 in the graph shown in FIG. 2 indicates the load characteristics of the LSI 3 in the key signal waiting status, whereas a line L2 indicates the load characteristics in the operation status. When illuminance is set corresponding to 150 Lux, a voltage of about -2.22 V corresponding to an intersection between the line L1 and a curve L4 is applied across the LSI 3. When the key signal is supplied to the LSI 3 which is then set to the operating status, a voltage of -1.93 V corresponding to an intersection between the line L2 and the curve L4 is applied across the LSI 3. In brief, when the key signal waiting status is changed to the operating status, the voltage of -2.22 V is changed to -1.93 V. However, the LSI 3 requires a minimum operating voltage V.sub.DDmin. If the voltage applied across the LSI 3 becomes lower than the minimum operating voltage V.sub.DDmin, the LSI 3 may operate erroneously or become inoperative. The voltage V.sub.DDmin is about -2 V for an LSI for a compact electronic calculator which has a rated operating voltage of -3 V. In order to operate the LSI which has the above-described voltage V.sub.DDmin using solar cells having the characteristics shown in FIG. 2, the LSI operates erroneously or becomes inoperative if illuminance is less than 150 Lux and hence the voltage applied across the LSI 3 is less than -2 V V.sub.DDmin. In the conventional arithmetic operation apparatus which has solar cells as the power source, when the intensity of light incident on the solar cells is made lower than a predetermined level, the apparatus operates erroneously or becomes inoperative.